1. Field of the Invention
The invention relates to a process of masking cooling holes of a gas turbine component.
2. Brief Description of the Related Art
Components such as gas turbine blades, vanes and other cooled parts often contain cavities that distribute cooling air to a plurality of holes in the wall of the part that lead to the outer surface. Most turbine components are coated for protection from oxidation and/or corrosion with, for example, a MCrAlY coating (base coat) and some are also coated with a thermal barrier coating (TBC) for thermal insulation. The demands of operation of the parts in a gas turbine often lead to the degradation of the coating before the structural integrity of the underlying part itself is degraded. Hence, the base coat and TBC must be removed and re-applied at least once during the lifetime of the component.
The re-application of the coatings can be very problematic for parts with a large number of cooling holes. Often the base coat can reach thicknesses of 150–300 μm, and the TBC may be another 200–500 μm in thickness. During original part manufacture, the coatings are usually first and then the holes are drilled directly through the coatings and the wall thickness of the component. However, during the repair operations the holes are already in place, including during re-coating. The combined thicknesses of these coatings would have a very significant (and negative) influence on the effectiveness of the cooling holes if the coatings were deposited into these holes during re-coating, especially considering that some holes are 1 mm or less in diameter. Specially shaped cooling holes are particularly susceptible to this as their effectiveness depends heavily on the accuracy of the shape of the hole. This problem is particularly great for the most modern components which contain hundreds of cooling holes and are designed to operate within very tight tolerance bandwidths—the upper limit on cooling hole diameter to stop the waste of un-needed cooling air (which drastically reduces engine efficiency and power output) and the lower limit on cooling diameter to prevent overheating of the component, which would lead to its failure in service. In fact, the filling of cooling holes can become so extreme that they are completely blocked, and it is even difficult to visually locate the cooling holes at all.
There have been several disclosures relating to this problem and there are several widely known practices. Those skilled in the art are aware that a common practice is to braze or weld the holes closed with a suitable material after the old coatings have been removed, re-apply the new coatings, and re-manufacture the holes. The problem with this is that the brazing or welding operations introduce zones of weakness into the material. Normal hole manufacturing operations have errors associated with the placement of the holes, and when residual welding or brazing material is left, the zones of weakness go into operation with the part and compromise the mechanical integrity of the part.
One disclosure which offers a solution to this is U.S. Pat. No. 5,702,288, in which an abrasive slurry is injected into the cavity of the component and forced through the cooling holes which were partially covered by the coating material. There was no welding or brazing closed prior to coating. However this also abrades the internal cooling configuration (ribs), any inserts, and also the non-coated portion of the cooling holes. In addition, it would not be possible to use this process on a stator vane which contained a cooling air distribution insert, without first removing the insert. This would be time consuming and very expensive. Another version of this technique is disclosed in U.S. Pat. No. 5,702,288. In these cases the abrasive slurry is injected from the outside of the component to the inside, through the cooling holes. However, the drawbacks of this method are similar, and there is an added problem of contamination of the coating with the slurry mixture. U.S. Pat. No. 5,702,288 also discloses the use of a masking agent for selectively choosing which cooling holes will be affected by the abrasive slurry. A further disadvantage of this method is that it would be nearly impossible to successfully use on shaped cooling holes, particularly on the continuously widening exterior portion due to the relatively very slow flow of slurry in that area.
Another disclosure which offers a better solution is U.S. Pat. No. 4,743,462, in which fugitive plugs are inserted into the cooling holes and partially volatilize during the coating process. The volatilization disrupts the coating in the region of the hole, and once the plugs are completely removed the holes are essentially free of coating and the cooling air will be unimpeded.
One disadvantage of the method disclosed in U.S. Pat. No. 4,743,462 is that the plugs must all be placed individually into the cooling holes. For small simple aeroengine parts such as the one illustrated in the disclosure (containing only several rows of cooling holes at the leading edge) this is feasible, however for large turbine components of land-based gas turbines which may contain several hundred cooling holes, it is no longer feasible to individually place plugs into each hole. This is further complicated by the fact that each component may be manufactured with several different types of cooling hole—including conical, straight cylindrical and holes with changing wall angles. Each type of cooling hole would require its own specially designed plug.
An alternative to this method is disclosed in U.S. Pat. No. 5,985,122 and U.S. Pat. No. B1-6,258,226, in which a tool is configured to fit simultaneously into a plurality of cooling holes prior to the application of coating (in this case using electrolytic coating techniques). The technique may be well suited for the protection of trailing edge cooling holes which are all aligned in one or two lines, but it would not be possible to use in thermal spraying coating techniques, particularly with components containing many cooling holes arranged along several rows on the airfoil, due to the “shadow” effect of the many required apertures to block the holes. A similar fixture is disclosed in U.S. Pat. No. 5,565,035.
A further disclosure in which all holes are plugged at once is given in U.S. Pat. No. 5,800,695. A masking agent is placed into the cooling configuration and forced through until it fills the cooling holes from the inside, but only up to the level of the exterior surface of the component. A coating is then applied, in this case electrolytically applied platinum. Due to the non-conductivity of the plastic maskant cited in the disclosure, no Pt would deposit on the masking agent in the cooling holes.
In addition, only plastic materials are specified as maskant materials, and in U.S. Pat. No. 4,743,462 the mask material is specified to volatilize at a temperature below that of the deposition process. The problem with this is that a part requiring a MCrAlY coating and TBC must have the MCrAlY coating “diffusion bonded” by a high temperature heat treatment (about 1000° C.–1150° C. in vacuum) before the TBC can be applied. These specified materials would not be retained for the TBC coating process, and would either have to be reapplied, or the advantage of the masking would be lost. Indeed, in patent U.S. Pat. No. 5,800,695 the example process clearly states that after electrolytic platinum coating, the maskant is removed and then the parts are aluminized, with no mention of protecting the cooling holes from Al deposition.
This problem is addressed in further current art which does not use plugging techniques. U.S. Pat. No. 6,004,620 discloses a technique in which the coating is applied to the component as normal, over the open and unprotected cooling holes. Then a high pressure water jet originating from the inside of the component cleans the cooling holes of unwanted coating build-up. This invention was originally destined for combustors—large conical components for which implementing this technique is feasible. However, it would be nearly impossible to insert a high pressure water jet device into the cooling configuration of a turbine blade. In stator vanes this only would be feasible, if the cooling air distributing insert is first removed.
A similar disclosure is given in US-A1-2001/0001680 and US-A1-2001/0006707. In this ease, it is specified that the coating is applied to the component at a special angle with respect to the angle of the cooling holes so as to minimize the amount of coating entering the cooling holes. In addition, the water jet still originates from the inside of the component (from the side with the uncoated surface) but the jet is aligned to be parallel with the axis of the cooling hole in order to more effectively remove the unwanted coating from inside the cooling hole. The water pressures used for the water jets in these applications range from 5 to 50 thousand pounds per square inch (psi) and in general the water does not contain abrasive particles.
Another possibility is disclosed in patent U.S. Pat. No. B1-6,210,488 in which a caustic solution is used to dissolve the TBC inside of cooling holes, assisted with ultrasound as an option. However, this disclosure was designed for complete removal of the TBC from a part coming out of service, and is not suitable for removing TBC only from inside the cooling holes. Masking the rest of the TBC over the entire outer surface of the component would not be practical.
U.S. Pat. No. 5,216,808 discloses the use of a pulsed ultraviolet laser to remove the unwanted TBC from within the cooling holes (after coating the component with unprotected cooling holes), since the wavelength of the UV laser is particularly well absorbed by the material used to make TBC's (namely zirconium oxide).
However, this method is not effective at removing unwanted MCrAlY in the cooling holes since the absorption of the radiation by the MCrAlY is not nearly as efficient as with the TBC. This invention recognizes the problem of extreme filling of the cooling holes with TBC material, as it includes the use of “machine vision” which locates the holes based on a datum in a CAD (computer aided design) file. A CNC positioning device then is used to properly locate the part relative to the laser, and then cleaning of the cooling holes is performed automatically according to the recorded positions of the cooling holes in the CNC data.
Another approach to the problem is to make the cooling holes larger than originally intended by design, and then coat directly onto the component without protecting the cooling holes. Since the cooling holes are too large to begin with, the coating which enters them will then bring them back within manufacturing tolerances and restore the desired cooling air flow. One version of this is disclosed in U.S. Pat. No. 6,042,879 in which an auxiliary coating is applied to the old coating, including that portion of the old coating adjacent to the cooling holes, and also to the base material of the cooling hole at the portion close to the exterior surface. This auxiliary coating is then diffused into the component with a heat treatment, and then the old and auxiliary coatings, together with the diffusion zone in the cooling hole outer portion are removed by chemical etching. A masking agent may also be used to protect the inner portion of the cooling hole from chemical attack. The disadvantage of this is that it requires a heat treatment at high temperature to create the diffusion zone, and those skilled in the art know that all such high temperature heat treatments degrade the microstructure of the alloy used for most gas turbine components. Furthermore, as admitted in the disclosure, not all the holes will be uniformly affected, but it is hoped that the overall distribution of cooling air flows will be within originally intended manufacturing tolerances (therefore this introduces uncertainty into the final temperature distribution of the component in the engine, which is highly undesirable).
So far the simplest solutions offered were those involving masking agents to prevent coating in the cooling holes, but these were only valid for electrolytic processes. However, U.S. Pat. No. B1-6,265,022 discloses the use of polymer based masking agents to be used for all types of coating processes in which the component did not have to be pre-heated to a temperature that would destroy the masking material. However, this disclosures specified that the masking material must protrude for a certain distance above the hole exit at the exterior surface of the component. Each disclosure had a different method of providing this: one by using a polymerizing energy source from the interior of the component, which is difficult to realize for many component designs and the other by providing a preform from wax, and then filing this with a polymerizing mask agent which, after hardening, provides the desired protrusions.